An Electrolyte with Elevated Average Valence for Suppressing the Capacity Decay of Vanadium Redox Flow Batteries

Nafion series membranes are widely used in vanadium redox flow batteries (VRFBs). However, the poor ion selectivity of the membranes to vanadium ions, especially for V2+, results in a rapid capacity decay during cycling. Although tremendous efforts have been made to improve the membrane’s ion selectivity, increasing the ion selectivity without sacrificing the proton conductivity is still a challenging issue. In this work, instead of focusing on enhancing the membranes’ ion selectivity, we develop an efficient valence regulation strategy to suppress the capacity decay caused by the crossover of V2+ in VRFBs. Despite the discharge capacity of the VRFB with the elevated average valence electrolytes (V3.68+) being slightly lower than that with commercial electrolytes (V3.50+) in the first 35 cycles, the accumulated discharge capacity in 400 cycles is improved by 52.33%. Moreover, this method is efficient, is easy to scale up, and provides deep insights into the capacity decay mechanism of VRFBs.

-Make a section for the electrolyte preparation.
-Change the metrics of GF pieces to cm if you further normalize the pump speed to it in cm2.
-Add a section to the description of the remixing procedures, especially include the value for the 'low discharge density'. Speaking of the discharge procedure here, why did not you use the constant voltage technique until the current stabilizes? At this point, you are having only crossover recharge and can tell the SOC is 0.
-Add a section for the viscosity measurements since you provide the experimental data.
-Add a description of Ox-C, Ox-4, Ox-0 meanings to pictures. Fig.S2, Fig.S4 -name the graphs with the same line names, add the name meanings in the description. 6) What is meant in 2.2 'At the initial state, both the vanadium ions contents and the valence of the electrolytes are symmetry for the positive and negative sides. Hence, all the concentrations of vanadium ions on the two sides are the same at 50 SOC.'? What is the chemical state described here as 50 SOC? This thesis is confusing because '50 SOC' state is, normally, an ideal case of half-charged battery having xV2+/xV3+ and 0V5+/ 0V4+ (zero) at the anolyte side vs xV5+/xV4+ and 0V2+/0V3+ (zero) at the catholyte side meaning different species concentrations on the two sides but equal concentration of each existing vanadium ion to each other; also this case excludes the preferential water transfer process. 7) Some additional information shall be added to S2, S4 figures because it is hard to tell the difference between them from the description. What is the role of the viscosity measurements for the research and why are the trends are different? 8) A table with all the settings and experimental data obtained shall be added. 9) Tab. S2: in order to make the available capacity calculations correct, you will need to change the metrics as (A h / L) as long as the system volume is not introduced and to add the formula explaining the values taken for the broad auditory of the journal that might not know the numbers used.
Also, consider using 'mol L-1' or 'M' through the article.
10) A part explaining the V2+ oxidizing process from the experimental setup perspective shall be added, especially in the case of Ox-C experiment. As for me, it raises a question of why did not all the V2+ oxidize? I would expect an accelerated capacity decay to a near-zero values in the case of a continuous air flow through the anolyte during the cycling.
11) The current density of 200 mA/cm2 is very high and reduces crossover significantly, while the research is exactly about this process. I would suggest such an experiment with 60 mA/cm current density, because it is hard to tell how close was the cycling to a symmetrical one from the SOC perspective, meaning asymmetric cycling, which is also a way of crossover control. In other words, the research might be considered as an addition to the asymmetric cycling procedure which makes the article very specific and would not reflect the general idea.
12) Make a general revision. I could have missed something as there are a lot of issues with the clearance.
Comments to the main article: 1) Fig.3: an information about the experiment shall be clarified: was it an experiment with initial V 3.5+? I would also suggest adding Ox-0, Ox-4, Ox-C meanings to the figure description.
2) Page 10, line 17: term 'cut-off voltage' describing the operating voltage might be confusing and I would suggest to change the term.
3) Fig.1(e): there is only V2+ crossover effect is demonstrated while vanadium ions of each valence have a significant effect on the process of V2+ surplus accumulation.
4) The theoretical explanation of the crossover effect on V2+ accumulation was accounted only at the 100% SOC and included only V2+ ions crossover, while there are four of them in total. Practically, the cycling was performed with only ~60-73% of the theoretical capacity depending on the answer to the 1 st question of the previous comments section. This means that 0 or 100% SOC have never even been achieved during the cycling experiment, in fact, it was in the range of ~15%/20% -75%/80% SOC meaning constant presence of V4+ ions having the second high diffusion coefficient after V2+ through the Nafion membrane [3] -this mechanism is also strongly affecting the process of V2+ accumulation. A more precise theoretical explanation of V 2+ accumulation is needed. Fig.4 (d-e): An oscillation is observed in the case of V 3.68+. An explanation shall be added for the process as it looks like if it was air-oxidized or shunt-connected, which would make the comparison incorrect. 6) Since there are 2 methods proposed to reduce the effect of V2+ accumulation, I would suggest to change the accent of the article to highlight the air oxidation method as well.

Dear Editors and Reviewers,
Thank you for your and reviewers' comments concerning our manuscript entitled "An Electrolyte with Elevated Average Valence for Suppressing the Capacity Decay of Vanadium Redox Flow Batteries" (oc-2022-01112j), which are very valuable and helpful for improving this paper. We considered these comments seriously and revised the manuscript based on them. The point-topoint responses to the comments are listed as follows: In this response document, text in italic style is the comments from the reviewers; text in regular style is our responses to the comments; revisions are highlighted in red color in both this letter, the  Firstly, oxidizing the accumulated V 2+ in the anolyte to V 3+ with air can increase the active species, which can be utilized in the following charge/discharge cycles, and improve the voltage efficiency of VRFBs ( Fig. 1(b)). The increased active species and voltage efficiency both contribute to the improvement of discharge capacity ( Fig. 3(a)).
Additionally, in this work, the maximum discharge capacity of the VRFB under 200 mA cm -2 is 1.822 Ah, which accounts for 64.76% of the theoretical capacity of the electrolytes (40 mL, 1.7 mol L -1 V 3.5+ ), and the proportion decreases with capacity decay. Therefore, the changing of active species reflected in the variation of anolyte concentration is much smaller than that reflected in the variation of discharge capacity. Furthermore, the concentration difference of V 3+ displayed in Fig. 1(c) is smaller than its actual value of the accumulated V 2+ . Because V 2+ is very easy to be oxidized by air, which results in part of V 2+ being oxidized to V 3+ during the UV tests. In fact, the actual concentration of accumulated V 2+ should be measured without oxygen. However, it is inevitable to interfere with the air during the UV sample preparation and measurement under our laboratory's experimental conditions. Besides, eliminating the accumulation of V 2+ can reduce the crossover of V 2+ from the negative to the positive sides and leads to the dynamic balance 1   Response: Thanks for your good advice. The schematic diagram of vanadium ions evolution of VRFBs with equilibrium (V 3.5+ ) and elevated valence electrolytes in the charge/discharge process are compared in Fig. S8. We also added the schematic diagram of vanadium ions evolution with the elevated valence of VRFBs in the revised manuscript, as shown in Fig. 4(h). The VRFB with the elevated valence electrolyte generates the surplus after a charge/discharge cycle ( Fig. 4(h)).   Response: Thanks for your valuable comments. The net flux of V 2+ changes with the charge/discharge process, which affects the active species ratio (V 2+ , V 3+ , VO 2 + , and VO 2 + ) and the maximum available capacity of VRFBs immediately. Let's assume that the anolyte of VRFB has 1 mol V 2+ , the corresponding catholyte has 1 mol VO 2 + at the initial stage, and the maximum discharge capacity is 1×F (F is Faraday constant F=96485 C mol -1 ). Next, we define x as the net flux of V 2+ from the anolyte to the catholyte after cycling for a certain time. In that case, the content of V 2+ in the anolyte for the fully charged VRFB is (1-x) mol, while the VO 2 + and VO 2+ contents in the catholyte are (1-2x) and 3x mol (the side reaction as Eq. (R1)). Hence, the maximum discharge capacity of the VRFB after x mol V 2+ net flux from the anolyte to the catholyte is (1-2x)×F. Moreover, 1 mol V 2+ consumes 1 mol VO 2 + in the discharge process. Therefore, it remains x mol V 2+ in the anolyte after fully discharging the VRFB, which is equal to the net flux of V 2+ crossover from the anolyte to the catholyte.
Therefore, in the VRFB, the net flux of V 2+ after running for a certain time is equal to the residual V 2+ in the anolyte after being fully discharged (discharge the VRFB with gradually decreased current densities). The optimal oxidized ratio of V 2+ is oxidizing all the accumulated unavailable V 2+ to V 3+ with air without sacrificing the available V 2+ . Thus, the optimal oxidized amount of V 2+ is equal to the residual amount of V 2+ in the anolyte after being fully discharged. That means we only need to fully discharge the VRFB and completely oxidize the anolyte with air without additional calculation in practice. (R1) On page 10, line 10, we find the wrong expression "V 3+ ". Please modify it.
Response: Thank you very much. We are very sorry for the typo. We have corrected it in the revised manuscript.
6. In Figure 4d and 4e, it seems that the discharge capacity and voltage efficiency of the VFB coupled with V 3.68+ electrolyte somehow exhibit regular fluctuations. Please explain it.
Response: Thanks for your valuable comments. The regular fluctuation of the discharge capacity and voltage efficiency of the VRFB coupled with V 3.68+ is caused by the regular variation in room temperature. In our laboratory, the temperature changes regularly with time, which influences the battery efficiencies and discharge capacity regularly. Additionally, the changing temperature effects are also reflected in the tests of VRFB coupled with V 3.50+ and present a similar fluctuation amplitude, as shown below. However, the fluctuation frequency with the cycle number of the VRFB coupled with V 3.50+ is much smaller than that coupled with V 3.68+ because the former displays a much smaller discharge capacity than the latter ( Fig. 4(d)). Therefore, the VRFB coupled with V 3.50+ runs more cycles at the same term than that coupled with V 3.68+ . Thus, the VRFB coupled with V 3.68+ presents more signific fluctuation with cycling than with V 3.50+ . We thank the reviewer for his/her important comments. We have carefully revised the manuscript according to these valuable suggestions.

Reviewer: 2
Recommendation: Reconsider after major revisions noted. Comments to the main article: 1. Fig.3: an information about the experiment shall be clarified: was it an experiment with initial in the same amount of unavailable accumulation in the catholyte, as shown in Fig. S3(b).
The results mean the accumulated amount of unavailable V 2+ is a constant at a certain net flux of electrolyte, no matter how much the vanadium ions (V 2+ , V 3+ , VO 2+ , ) contribute to the + 2 VO crossover separately. Moreover, the concentration and volume of the electrolyte increase on the positive side and decrease on the negative side with cycling 1 due to the much higher diffusion rate across the Nafion series membranes of V 2+ than that of other vanadium ions. 2 The electrolytes' changing during VRFB cycling results in the surplus of V 2+ on the negative side. 3,4 Therefore, in this work, we used the net flux of V 2+ (the amount of vanadium ions (V 2+ /V 3+ ) crossover from the anolyte to the catholyte, subtract the amount of vanadium ions (VO 2+ /VO 2 + ) crossover from the catholyte to the anolyte) to depict the effects of vanadium ions crossover on the accumulation process of unavailable V 2+ for brevity. We also modified the related contents in the revised manuscript to make it more straightforward.
Side reactions for the vanadium ions transport from the anolyte to the catholyte: (S1) Side reactions for the vanadium ions transport from the catholyte to the anolyte:    In practice, the capacity of a VRFB with a certain amount of electrolyte is influenced by the electrolyte flow rate, current density, and other working conditions, which results in the capacity being lower than the electrolytes' theoretical capacity. In this work, the theoretical capacity of the electrolytes (40 mL, 1.7 mol L -1 V 3.5+ ) is 1.822 Ah, and the maximum discharge capacity of the VRFB in the experiment is 1.180 Ah (64.76% SOC).
As the reviewer comments, the SOC of VRFB affects the vanadium ions crossover 5 and influences the V 2+ accumulation in the charge/discharge process. However, as mentioned above, the accumulated amount of unavailable V 2+ is a constant at a certain net flux of the electrolyte, no matter how much the vanadium ions (V 2+ , V 3+ , VO 2+ , ) contribute to the crossover separately.

Fig.4 (d-e): An oscillation is observed in the case of V 3.68+ . An explanation shall be added for the process as it looks like if it was air-oxidized or shunt-connected, which would make the comparison incorrect.
Response: Thanks for your valuable suggestion. The regular fluctuation of the discharge capacity and voltage efficiency of the VRFB with V 3.68+ electrolyte is caused by the regular variation in room temperature. In our laboratory, the temperature changes regularly with time, which influences the battery efficiencies and discharge capacity to some extent. Additionally, the changing temperature effects are also reflected in the tests of VRFB with V 3.50+ electrolyte and present a similar fluctuation amplitude, as shown below. However, the fluctuation frequency with the cycle number of the VRFB with V 3.50+ is much smaller than that with V 3.68+ because the former displays a much smaller discharge capacity than the latter (Fig. 4(d)). Therefore, the VRFB with V 3.50+ electrolyte runs more cycles under the same term than that with V 3.68+ electrolyte. Thus, the VRFB with V 3.68+ electrolyte presents more signific fluctuation with cycling than with V 3.50+ electrolyte. We have added an explanation of the oscillation phenomenon in the revised manuscript. [In the revised manuscript, page 15, line20] Besides, the discharge capacity and voltage efficiency of the VRFB coupled with V 3.68+ present a larger fluctuation frequency with cycling than that of VRFB coupled with V 3.50+ , but they display a similar fluctuation amplitude. The reason for the fluctuation is the varying room temperature, which changes regularly with time in our laboratory. However, the discharge capacity of the VRFB coupled with V 3.50+ is much smaller than that with V 3.68+ after 200 cycles (Fig. 4(d)), leading to a much smaller fluctuation frequency with cycles for the former. Moreover, the rapid capacity decay of the VRFB with V 3.50+ in the first 150 cycles also concealed the fluctuation phenomenon.

Since there are 2 methods proposed to reduce the effect of V 2+ accumulation, I would suggest
to change the accent of the article to highlight the air oxidation method as well.
Response: Thanks for your important suggestion. According to your suggestion, we highlighted that oxidizing the unavailable V 2+ to V 3+ with air could significantly improve the discharge capacity of VRFBs in the conclusion section. However, the inside of the VRFB system is generally isolated from the external environment to ensure the sealing system and avoid additional side reactions in practical applications. Therefore, although both methods can effectively reduce the capacity decay rate of VRFBs, the utilization of electrolytes with an elevated average valance is a preferred method.
Moreover, the air oxidization method proposed in this work is mainly used to demonstrate the effects of the accumulated unavailable V 2+ on the capacity of VRFBs and introduce the electrolytes with elevated average valence. In short, we think elevating the average valence of electrolytes is more meaningful and easier to handle for the engineering application of VRFBs.
Comments to the experimental section of the supporting information:

From the supplementary materials it is not clear, what electrolyte was used for the V 3.68+ tests (Fig 4. (d-g) of the main article): was it the same initial electrolyte for both anolyte and catholyte or was it V 3.68+/ V 3.5+ pair?
Response: Thanks for your suggestion. In this work, the V 3.68+ electrolytes were used as the catholyte and anolyte of VRFBs in the battery tests (noted as V 3.68+ ). We have added the detail description of this section in the revised supporting information (Section 1.6).
2. Tab S1: the table shown originally [3] was named as for the Nafion 115 membrane, not for 'Nafion serials membrane', and the diffusion coefficient was normalized to the cm 2 , not cm 1 with the same quantitative values -this should be corrected or additional comments should be added.
Even though those parameters were measured for Nafion 115 membrane, they were normalized to the membrane`s surface area, not the thickness, and direct application of these parameters to a twice thicker Nafion NR212 membrane is questionable.
Response: Thanks for your good comments. I am sorry for the typo on the diffusion coefficient.
We have corrected the metric of diffusion coefficient, as well as the title of Tab. S1 in the revised supporting information. Additionally, the diffusion coefficient of vanadium ions is obtained in the reference of [3] as below:  Let's assume that the total volume of the anolyte and catholyte in VRFBs is the same before and after cycling. However, the electrolytes cannot wholly be pumped out from the VRFB after cycling due to the electrode absorption and the residual in pipes and channels, resulting in the total electrolytes after cycling obviously less than that at the initial stage, as shown in Fig. S4 Hence, the total vanadium ion in the electrolyte of VRFB after 400 cycles can be calculated as Eq.
(S12). And the calculated result is very close to that at the initial stage.
(S12) We have also added these descriptions in the revised supporting information. Fig. S7 is named as (1), while the calculation after Fig. S1 is not -why?

The calculation after
Response: Thanks for your suggestion. We have numbered all the equations in the revised supporting information.

Please, name all the sections and the description correctly.
Response: Thanks for your suggestion. We have renamed the related sections in the revised supporting information.
Response: Thanks for your suggestion. We have replaced 'Characteristics' with UV-vis spectroscopy in the revised supporting information.

-Make a section for the electrolyte preparation.
Response: Thanks for your suggestion. We have added Section 1.3, which was named electrolyte preparation, and described the electrolyte preparation process more specifically in the revised supporting information. Details as below: [In the revised supporting information, Section 1. Response: Thanks for your suggestion. We have added a Section 1.4 named 'Remixing electrolytes' in the revised supporting information, details as below: In this work, we used Arbin 179539 (Arbin instruments, USA) to record the experiment data, which has a maximum working current of 10 A. However, the discharge current exceeds 10 A at the initial stage when we use the constant voltage model to discharge the VRFB. Therefore, in this work, we adopted the gradient decreasing current density (312. Step 1: Step 2: Step 3: Step 4: Catholyte Anolyte Catholyte Anolyte Catholyte Anolyte Catholyte Anolyte Inlet Outlet Fig. S1. Remixing electrolytes procedures.
12. -Add a section for the viscosity measurements since you provide the experimental data.
Response: Thanks for your suggestion. We have added a Section 1.5 named 'Viscosity measurement' in the revised supporting information, details as below: [In the revised supporting information document, Section 1.5, Page S2, line 18] The viscosity of vanadium electrolytes is tested with the 1834 Ubbelohde viscometer in a thermostat HS0100 (Lorderan, China).
13. -Add a description of Ox-C, Ox-4, Ox-0 meanings to pictures. Fig.S2, Fig.S4 Response: Thanks for your suggestion. The Ox-0 presents the VRFB operated with nitrogen protection without V 2+ oxidized by air during 400 cycles. Ox-4 means the VRFB with 4 times air oxidization of V 2+ at the 50th, 100th, 200th, and 300th cycles during cycling. Ox-C represents the VRFB in which the anolyte is oxidized continually with air during cycling. We have added the related description in the revised supporting information.
14. -name the graphs with the same line names, add the name meanings in the description.
Response: Thanks for your suggestion. We have renamed the legends in graphs and made them consistent. The description of the related legends is also added in the revised supporting information. VRFBs, the crossover is an accumulated process and displays a slight effect on the capacity in one cycle (the coulombic efficiency of VRFB is larger than 98%, the corresponding capacity retention rate per cycle in 100 cycles is 99.53% based on Figs. 3(a-b)). Therefore, we can neglect the crossover effects when charging the VRFB from 0 to 50 SOC. It is to say, the content and concentration of the vanadium ions in the VRFBs are the same at 50 SOC (contents: V 3+ = VO 2 + = V 2+ =VO 2 + =0.5×40×1.7 mM). In this work, we did not consider the water transfer effects on capacity decay. Moreover, just as the response to Question 3, we have modified the description of Section 2.2 in the revised supporting information.

What is meant in 2.2 'At the initial state, both the vanadium ions contents and
[In the revised supporting information, Section 2.1, Page S5, line 14] The side reactions (Eqs. S1-S6) will occur when vanadium ions cross the membrane from one side to the other side, and the effect of crossover on vanadium ions' evolution in the charge/discharge process is depicted in Fig. S3. As shown in Fig. S3(a), although the crossover of V 2+ and V 3+ consumes different amounts of in the catholyte, both result in the same amount (S1)

Some additional information shall be added to S2, S4 figures because it is hard to tell the difference between them from the description. What is the role of the viscosity measurements for the research and why are the trends are different?
Response: Thanks for your suggestion. Increasing the electrolytes' concentration leads to a high electrolyte viscosity. Therefore, the viscosity measurements are used to auxiliary demonstrate the concentration changes with cycling.
To be specific, Fig. S5 is used to demonstrate that the vanadium ions' concentration of the VRFB with V 3.5+ electrolyte varies with cycling which is also reflected in the change of electrolytes' viscosity. Fig. S7 is used to present the increasing hydraulic gradient between the two sides of the membrane with cycling. The hydraulic gradient drives the net flux of the active species transport from the catholyte to the anolyte and leads to the discharge capacity of Ox-C increasing with cycling after 300 cycles (Fig. 3(c)). We have added the related description under Fig. S2 and Fig.   S4 in the revised supporting information. Details as below： [In the revised supporting information, Section 3, Page S12, line 10] Fig. S5 shows that the viscosity of electrolytes in the catholyte increases rapidly in the first 100 cycles and then slows down the increasing rate, which is consistent with the change in electrolytes' concentration with cycling. Similarly, the viscosity of electrolytes in the anolyte presents the same trend as that of the electrolytes' concentration change with cycling.
[In the revised supporting information document, Section 3, Page S13, line 14] In Fig. S7, Ox-C represents the VRFB in which the anolyte is continually oxidized with air during cycling. Fig. S7 shows that the gap of viscosity between the anolyte and catholyte of Ox-C increases with cycling, which increases the amount of vanadium ions transport from the catholyte to anolyte by convection and diminishes the lack of V 3+ in the anolyte. Thus, the capacity of Ox-C increases with cycling after 300 cycles due to the increment of active species in the anolyte. The one is oxidizing V 2+ after a full discharge (Ox-4), and the other is oxidizing V 2+ with air continually during the charge/discharge cycling (Ox-C). In the first method, a gradient decreasing current density (312.5-200-125-62.5-50-18.75 mA cm -2 ) is applied to discharge the VRFBs to 0 SOC and avoid excessive oxidization of V 2+ . Then, blow air into the anolyte tank, ensure the pipe is inserted into electrolytes ( Fig. S2(a)) to accelerate the oxidization process and ensure V 2+ is oxidized completely. Moreover, a sealing film (PARAFILM@, USA) is used to avoid the electrolyte splash. This process continues for 6 hours to ensure the oxidization of V 2+ with air completely. For the continued oxidization of V 2+ with air, we just loosen the sealing cap of the anolyte tank and allow air exchanges slowly inside and outside the tank after the oxygen in the tank is consumed by V 2+ , as shown in Fig. S2(b). We have added Section 1.7, named Oxidizing electrolytes, in the revised supporting information. Detailed is shown below. In this work, oxidizing the unavailable V 2+ with air is used to increase the active species of V 3+ and make the content of V 3+ closer to that of VO 2+ . Hence, the VRFB cannot discharge if we oxidize all V 2+ with air due to the lack of V 2+ in the anolyte. Meanwhile, the VRFB cannot charge due to the lack of VO 2+ in the catholyte, as shown in Fig. R2. Therefore, the discharge capacity is sharply down to 0 when we oxidize all V 2+ with air (Fig. R2). Notably, a 50 mA cm -2 is applied on the VRFB before oxidizing the V 2+ with air to ensure most of the V 2+ is oxidized by air.  As shown in Fig. R2, the capacity decay will be profoundly accelerated if excessive V 2+ is oxidized with air during the charge/discharge process. Therefore, an experiment with compressed air blowing into the anolyte tank continually during the charge/discharge process is conducted, as shown in Fig. R3(a).

A
In Figs. R3(b-c), V 3.50+ means the VRFB with conventional electrolyte (V 3.50+ ) and carries out the charge/discharge with N 2 protection, and V 3.50+ -air means the VRFB with conventional electrolyte (V 3.50+ ) and carries out the charge/discharge with N 2 protection in the first 30 cycles and then blow the compressed air into the anolyte tank during the charge/discharge cycling. Fig. R3(b) shows that V 3.50+ presents the same trend in the discharge capacity as that of V 3.50+ -air in the first 30 cycles.
However, the discharge capacity of V 3.50+ -air increases in a few cycles after blowing compressed air into the anolyte tank because the accumulated unavailable V 2+ is oxidized to V 3+ , which is consistent with this work. Then, the discharge capacity rapidly decreases after 35 cycles due to the excessive oxidization of V 2+ with air. The excessive oxidization of V 2+ with air decreases the content of V 2+ in the discharge process and results in lower coulombic efficiency and voltage efficiency, as shown in Fig. R3(c). In Fig. R3(c), the rise of voltage efficiency at 63 cycles is caused by the minimal capacity (close to 0), which results in a significant fluctuation. The fluctuation is also reflected in the coulombic efficiency of V 3.5+ -air, as shown in R3(c).

Oxidizing V 2+ with air
This work used two different ways to oxidize V 2+ with air. One is oxidizing V 2+ after a full discharge (Ox-4), and the other is oxidizing V 2+ with air continually during the charge/discharge cycling (OX-C). In the first method, a gradient decreasing current density (312.5-200-125-62.5-50-18.75 mA cm -2 ) is applied to discharge the VRFBs to 0 SOC and avoid excessive oxidization of V 2+ . Then, blow air into the negative electrolyte tank and ensure the pipe is inserted into electrolytes ( Fig. S2(a)) to accelerate the oxidization process and ensure V 2+ is oxidized completely.
Moreover, a sealing film (PARAFILM@, USA) is used to avoid the electrolyte splash. This process continues 6 hours to ensure the oxidization of V 2+ with air completely. For the continued oxidization of V 2+ with air, we just loosen the negative tank's sealing cap and allow air exchanges slowly inside and outside the tank after the oxygen in the tank is consumed by V 2+ , as shown in  20. The current density of 200 mA/cm2 is very high and reduces crossover significantly, while the research is exactly about this process. I would suggest such an experiment with 60 mA/cm current density, because it is hard to tell how close was the cycling to a symmetrical one from the SOC perspective, meaning asymmetric cycling, which is also a way of crossover control. In other words, the research might be considered as an addition to the asymmetric cycling procedure which makes the article very specific and would not reflect the general idea.
Response: Thanks for your question. As a most promising flow battery energy storage technology, VRFB is very close to commercialization. Till now, the operating current density of VRFBs is 150-300 mA cm -2 . 1 200 mA cm -2 is a typical parameter widely used to estimate the battery performance in single cell 2,3 and kW scale stacks. 4,5,6 Therefore, we investigated the cycling performance of VRFBs with the proposed method under 200 mA cm -2 in this work.
The current density significantly affects the operating SOC of VRFBs in the charge/discharge process, which also influences the capacity decay in round charge/discharge cycling. 7 However, as the response to Question 3, although the crossover of V 2+ and V 3+ consumes a different amount of on the positive side, both result in the same amount of unavailable V 2+ on the negative + 2 VO side finally (Fig. S3(a). Similarly, the crossover of VO 2+ and consuming different amounts of V 2+ on the negative side result in the same amount of unavailable on the positive side, as shown in Fig.   S3(b). That means the accumulated amount of unavailable V 2+ is a constant at a certain net flux of electrolyte, no matter how much the vanadium ions (V 2+ , V 3+ , VO 2+ , ) contribute to the + 2 VO crossover separately. Moreover, the concentration and volume of the electrolyte increase on the positive side and decrease on the negative side with cycling 8 is a general result and was widely